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Titel:

Due to the request for lower weight and lower costs for future space applications, MT Aerospace (MT) is developing forward-looking manufacturing processes for a new generation of composite pressure vessels and structural launcher components. Together with the Chair of Carbon Composites (LCC) MT developed a manufacturing process for the in situ connection of aluminum profiles with carbon fiber reinforced thermoplastics (CFRTP). At the end of the project a cylindrical demonstrator was manufactured at LCC and tested afterwards at the German Aerospace Center (DLR). The process is based on a Thermoplastic Automated Fiber Placement (TP-AFP) process where carbon fiber reinforced tape materials are directly joined with the aluminum substrates. Joining with the metal and consolidation of the laminate is realized in one process step, process cost and time consuming joining steps like bonding or riveting processes are avoided. Before joining, the interface areas of the aluminum profiles need to be pretreated to reach an optimal adhesion between the thermoplastic matrix and the aluminum alloy. During a study phase different surface treatments have been assessed concerning the joint strength and the influence of the surface treatment on the material properties of the aluminum. In the end of the study the best suited surface treatment was chosen for manufacturing of the test hardware. To demonstrate the feasibility and performance of the joining technology on demonstrator level, a reinforced cylinder with integrated aluminum stringers and a skin made out of CFRTP has been designed and manufactured. The pre-dimensioning was performed using the MT software ODIN. Additionally non-linear buckling analyses have been performed. The cylinder has a diameter of 800 mm and a length of 1200 mm with a laminate thickness of one millimeter. At the inner surface 18 aluminum stringers are attached using the developed process. To increase the circumferential stiffness, three CFRTP stiffeners are wound on the outer surface. After an ultrasonic testing and an inspection of geometrical surface imperfections by photogrammetry the demonstrator was tested under axial compression. During the test the deformations were recorded using the digital image correlation system ARAMIS. At the maximum load of 325 kN global buckling occurred. The failure load was in the predicted range and the buckling pattern correlates well to the prediction of the non-linear simulation. The evaluation showed that both before and after occurrence of the global buckling the axial strains in the skin and in the joining area are homogeneous. After unloading the buckling pattern was remaining in the demonstrator which is caused by the plastic deformation of the stringers. No separating of the stringers from the skin could be detected which indicates an excellent bonding quality between stringer and skin. Based on the results of this study MT and TUM will continue working on the in situ joining of metals and CFRTP.